This invention relates to capacitive sensors, and more particularly to variable capacitance sensors for determining relative position or movement between objects, such as movement of liquid within a tank, movement of one machine component with respect to another machine component, as well as other measurable parameters.
Transducers for measuring liquid level, linear or angular position and movement, pressure, torque, and the like, are often used in vehicles, industrial equipment and other systems and components. The electrical output of such transducers changes in response to a change in the component being measured. The electrical output is typically in the form of a change in resistance, capacitance, current flow, magnetic field, and frequency. These types of transducers may include variable capacitors or resistors, optical components, Hall Effect sensors, strain gauges, ultrasonic devices, and so on.
By way of example, prior art liquid level sensors, such as fuel sensors for motor vehicles, usually include a float that rides on an upper surface of the fuel in a fuel tank. The float is typically connected to one end of a pivot arm while the other end of the pivot arm typically includes a wiper mechanism that brushes against a resistor strip when the arm is rotated due to a change in fuel level in the tank. Such sensors are prone to wear, mechanical and/or electrical breakdown or inaccurate liquid level detection. Although variable capacitance probes have been developed to overcome these drawbacks, they are cost-prohibitive in many applications and are typically limited to measuring a certain type of liquid, since different liquids will have different dielectric properties.
In addition, a variable capacitance probe designed to measure fuel level normally cannot be used for measuring water level due to the different dielectric properties associated with different liquids. For example, the dielectric constant at room temperature of a vacuum is one, of air is close to one, of gasoline is about two, of industrial alcohol is anywhere from 16-31, and of water is about 80. Since capacitance is directly dependent on the dielectric constant, a transducer designed for measuring the level of one type of liquid could not be relied upon for measuring other types of liquids. However, even when the transducer is designed for measuring only one type of liquid, such as gasoline, the dielectric constant can change due to different gasoline formulations, the presence of water, alcohol, detergents, additives, as well as environmental factors such as temperature, pressure and humidity, thus leading to significant measurement inaccuracies.
In order to overcome these challenges, the prior art has suggested compensation means in the form of a reference capacitor at the bottom of the tank, which must always be immersed in the liquid being measured. The reference capacitor includes a pair of spaced plates and the liquid being measured serves as the dielectric between the plates. A dielectric constant of the liquid can then be determined and used to offset the capacitance of the liquid level sensor to compensate for dielectric variations. However, this type of solution only takes into account changes in dielectric at the bottom of the tank since it is assumed that the liquid is uniform throughout it's volume. In reality, since some liquids have a greater density than others, there may be a separation of fluids at different depths that cannot be discerned by a single reference capacitor. For example, the reference capacitor may be exposed to water or other contaminants at the bottom of a fuel tank which may lead to inaccurate dielectric compensation.
In many applications, such as all-terrain vehicles, motorcycles, snowmobiles, and the like, tanks are typically formed with curved side walls to fit around the engine and/or other components of the vehicle. In such circumstances, prior art fuel sensors are usually inadequate to accommodate these shapes while accurately measuring fuel level. In addition, many tanks are constructed without the provision of openings or mounting hardware for accommodating a liquid level sensor. In other circumstances, it may be difficult or impractical to penetrate the tank with a liquid level sensor.
In addition, the electronics associated with capacitive measurement and compensation are relatively expensive and are thus priced out of markets where there is a long-felt need for low-cost yet highly accurate liquid level transducers and other sensors for measuring relative position and/or movement between objects.